9s CENTROLECITHAL SEGMENTATION. 



the nuclei are so often double that I have no doubt they are rapidly 

 undergoing division. It appears to me probable that, at the time 

 when the superficial layer of protoplasm is segmented otf from the 

 yolk below, the nuclei undergo division, and that a nucleus with 

 surrounding protoplasm is left with each yolk column. For further details 

 vide Chapter on Arachuida. 



Although by the close of the segmentation the protoplasm has 

 travelled to a superficial position, it may be noted that at first 

 it forms a small mass in the centre of the egg, and only eventually 

 assumes its peripheral situation. It is moreover clear that in the 

 Spiders' ovum there is, so to speak, an attempt at a complete segmen- 

 tation, which however only results in an arrangement of the constitu- 

 ents of the ovum in masses round each nucleus, and not in a true 

 division of the ovum into distinct segments. 



It seems very probable that Ludwig's observations on the segmentation 

 of Spiders only hold good for species with comparatively small ova. 



In connection with the segmentation of the Insects' ovum and allied 

 types it should be mentioned that Bobretzky, to whose observations we are 

 largely indebted for our knowledge of this subject, holds somewhat different 

 views from those adopted in the text. He regards the nuclei surrounded by 

 protoplasm, which are produced by the division of the primitive segmenta- 

 tion nucleus, as so many distinct cells. These cells are supposed to move 

 about freely in the yolk, which acts as a kiud of intercellular medium. This 

 view does not commend itself to me. It is opposed to my own obser- 

 vations oil similar nuclei in the Spiders. It does not fit in with our know- 

 ledge of the nature of the ovum, and it cannot be reconciled with the seg- 

 mentation of such types as Spiders or even Eupagurus, with which the seg- 

 mentation in Insects is undoubtedly closely related. 



The majority if not all the cases in which a central yolk mass is formed 

 occur in the Arthropoda, in which group centrolecithal ova are undoubtedly 

 in a majority. In Alcyonium palmatum the segmentation appears however 

 to resemble that of many insects. 



One or two peculiar varieties in the segmentation of ova of this type 

 may be spoken of here. The first one I shall mention is detailed in the 

 important paper of E. van Beneden and Bessels which I have already so 

 often had occasion to quote : it is characteristic of the eggs of most of the 

 species of Chondracanthus, a genus of parasitic Crustaceans. The ovum 

 divides in the usual way but somewhat irregularly into 2, 4, 8 segments 

 which meet in a central yolk mass but after the third division instead of 

 each segment dividing into two equal parts it divides at once into four, and 

 the division into four having started, reappears at every successive division. 

 Thus the number of the segments at successive periods is 2, 4, 8, 32, 128, 

 etc. In another peculiar case, an instance of which 1 is afforded by Asellus 

 aquaticus, after each of the earlier segmentations all the segments fuse and 

 become indistinguishable, but at the succeeding segmentation double the 

 number of segments appears. 



Although, as has been already stated, it does not seem possible to have 

 a true meroblastic segmentation in centrolecithal ova, it does nevertheless 

 appear probable that the apparent cases of a meroblastic segmentation 



1 Ed. van Beneden, Bull. Acad. Behjiquc, Vol. xxvni. 1869. 



